I have recently eaten at Sushi Zone (on Market between Valencia and Guerrero or is it Dolores?) right by It's Tops diner...although the wait is horrendous, I would have to say that each time I've eaten there (3 x's now) I have had incredibly fresh, super tasty, huge nigiri sushi. Great special rolls, too.
I would highly recommend this place to anyone who wants to feel like they've really found a "find". But, be warned, there are 10 stools and 2 booths which means that unless you show up there early, you're going to wait for an hour.
Prices are super reasonable...I went with friends and in addition to stuffing ourselves with sushi, we had beer and sake with dinner and the damage was only $24/person...but they only take cash.